1. Field of the Invention:
The present invention relates to a noise reducing circuit using a clamping circuit to reduce noises in a low frequency band.
2. Description of the Prior Art:
Description will be made herein by way of an example of a circuit used for reducing noises in a low frequency band of video signals obtained from a solid state image sensor.
While developments of video cameras using a solid state image sensor as an image pick up means are widely known, there still remain a number of disadvantages to be overcome. One of the disadvantages, the sensitivity of a camera, wherein low band noises at the photographing time with low intensity illumination constitute a difficulty in improving the sensitivity. In general, a light shielding layer called an optical black is provided for an image sensor and a video signal corresponding to the light shielding layer is used as an optical reference black signal. A method to compensate a dark current of the image sensor by effecting clamping, using the reference black signal as a reference clamping signal, has been employed.
On the other hand, when a video signal corresponding to an incident light beam is read from an image sensor, the video signal has noise, which is generated from an on-chip amplifier for signal charge detection (hereinafter called "output amplifier") being provided in one chip at an output of a solid state image sensor, added thereto.
For example, when a MOS-FET is used in an output amplifier, a low frequency noise characteristic of the MOS-FET is characterized by such low band noise characteristics shown by the solid line 1 in FIG. 1. The noise characteristic changes at a frequency f.sub.A which may be spotted at several tens of KHz to several hundreds of KHz. Then noise increases as the frequency lowers to a band lower than the frequency f.sub.A. Such noise at a low band has the characteristic of being in inverse proportion to the frequency, hence called 1/f noise.
An explanation shall be made as to why a level of noise in a low band contained in a signal is reduced by a clamping circuit. Now a structure is referred to wherein one element of noise (angular frequency W.sub.h) is contained in a signal. A ratio .DELTA..sub.h of noise reduction in a clamping circuit is represented by the following equation, in which the frequency of a clamping pulse is expressed as T.sub.H : EQU .DELTA..sub.h =W.sub.h T.sub.h.
And if, for example, the above is substituted with T.sub.H =63.5 .mu.s which is a horizontal scanning period of a television and .DELTA..sub.h =1, an upper frequency limit which can reduce noise will be about 2.5 KHz.
Here, if the upper frequency limit is represented by f.sub.L, and a reference clamping signal containing the above-mentioned 1/f noise is clamped, the distribution of noise will be flat at the frequency f.sub.L or lower. Therefore, when the level of a low band noise is large, the low band noise will be reduced.
Now, a method to reduce the 1/f noise by employing the above-mentioned nature of a clamping circuit may be conceived.
That is, when 1/f noise is passed through a clamping circuit, the noise level will become flat at the threshold frequency f.sub.L as shown by the broken line 2 in FIG. 1, thus low band noises are reduced.
However, since a low band noise has a higher noise level than that of a high band noise by a few to several dB, the low band noise is stressed in photographing with low intensity illumination which is represented by the line-shape noise in a television receiver plane, creating very unsightly video images.
It is an object of the present invention to provide a noise reduction circuit which can reduce with improved efficiency a low band noise by a clamping circuit. Also, it is another object of the present invention to provide an image pick up apparatus which can obtain a video signal in which a low band noise has been satisfactorily reduced. Further objects of the present invention will become clear by the detailed description of preferred embodiments with reference to the attached drawings.